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To: cygwin@cygwin.com
Subject: unhandled C++ exceptions not propagating
Date: Sat, 15 Aug 2020 20:38:01 -0300
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From: David McFarland via Cygwin <cygwin@cygwin.com>
Reply-To: David McFarland <corngood@gmail.com>
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I was just debugging a c++ app (b2 build system from boost), and noticed
that it would appear to exit unexpectedly without an error.  This turned
out to be when an unhandled C++ exception was thrown.

On a fresh install of cygwin with gcc-g++, this program will throw an
exception from the std::string constructor:

c++ -x c++ - <<END
#include <string>
int main() { std::string str(nullptr); return 0; }
END

When it's executed from the shell it returns zero, but execution stops
at the exception.

$ ./a; echo $?
0

When executed under gdb, the exception is caught, and the process exits
non-zero when continued:

(gdb) r
Starting program: /home/corngood/a
[New Thread 4300.0x1390]
[New Thread 4300.0x1d24]
[New Thread 4300.0x1d48]
[New Thread 4300.0x80c]
gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
Thread 1 "a" received signal ?, Unknown signal.
0x00007ff8d2cfa799 in RaiseException () from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
(gdb) c
Continuing.
[Thread 4300.0x1d24 exited with code 541541187]
[Thread 4300.0x1870 exited with code 541541187]
[Thread 4300.0x1d48 exited with code 541541187]
[Thread 4300.0x1390 exited with code 541541187]
[Inferior 1 (process 4300) exited with code 04021641503]
(gdb)

When executed under strace, it exits with an error as expected:

$ strace -o /dev/null a; echo $?
67

That's as far as I've investigated so far.
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